This invention relates generally to a method for fabricating a fully integrated (monolithic) inkjet printhead, and more particularly to a method for forming pillars within the printhead to reduce particle clogging of ink refill channels.
A thermal inkjet printhead is part of an inkjet pen. The inkjet pen typically includes a reservoir for storing ink, a casing and the inkjet printhead. The printhead includes a plurality of nozzles for ejecting ink. A nozzle operates by rapidly heating a small volume of ink in a nozzle chamber. The heating causes the ink to vaporize and be ejected through an orifice onto a print medium, (e.g., a sheet of paper). Properly sequenced ejection of ink from numerous nozzles arranged in a pattern causes characters, symbols or other graphics to be printed on the print medium as the printhead moves relative to the print medium.
The inkjet printhead includes one or more refill channels for carrying ink from the reservoir into respective nozzle chambers. According to one conventional fabrication methodology, a nozzle chamber is defined in a barrier layer applied to a substrate. An orifice plate is applied to the barrier layer. The substrate forms a floor of the firing chamber (along with a firing resistor), while the orifice plate forms a ceiling to the firing chamber. According to another conventional fabrication methodology, a fully integrated, or monolithic, printhead of inkjet nozzles is formed using photoimaging techniques similar to those used in semiconductor device manufacturing. The fully integrated thermal (FIT) inkjet printhead includes a thin film layer formed of various passivation, insulation, resistive and conductive layers applied to a silicon wafer.
One problem which affects print quality is clogging of the ink refill channels. Once a nozzle chamber is fired ejecting a drop of ink, ink flows from the reservoir through the ink refill channels into the nozzle chambers. Typically, the ink is stored within a porous material filling the reservoir to achieve fluid retention and fluid pressure benefits. A disadvantage of the porous material, however, is that particles are occasionally disengaged and carried by the ink into the ink refill channels. Even for devices without a porous material in the ink reservoir, particles remaining from manufacturing processes may be carried by ink to the refill channels. Such porous material particles or leftover manufacturing process particles can become lodged and block a refill channel. Blocking of a refill channel can cause premature failure of an inkjet firing chamber, or cause ink starvation of the inkjet firing chamber. The failure of a nozzle to eject an ink droplet can harm print quality. Redundant nozzles have been proposed and implemented as one solution to this problem.
Pillars and barrier islands have been proposed to capture particles and provide redundant pathways leading to the nozzle chambers. U.S. Pat. No. 5,463,413 issued Oct. 31, 1995 to Ho et al. for xe2x80x9cInternal support for Top-Shooter Thermal Inkjet Printheadxe2x80x9d discloses pillars for a printhead formed by a substrate, barrier layer and orifice plate. U.S. Pat. No. 5,734,399 issued Mar. 31, 1998 to Weber et al. for xe2x80x9cParticle Tolerant Inkjet Printhead Architecturexe2x80x9d discloses barrier islands for a printhead also formed by a substrate, barrier layer and orifice plate. Both of these patents disclose forming the pillars or barrier islands in the barrier layer before applying the orifice plate.
According to the invention, pillars are formed in a fully integrated thermal inkjet printhead to prevent particles from entering into a nozzle chamber along an ink refill channel. Ink can flow into the nozzle chamber even in the presence of a particle blocking one of multiple ink refill channels leading to the nozzle chamber.
According to one aspect of the invention, the pillars are formed after a step of applying a thin film structure to a printhead substrate. The thin film structure includes various passivation, insulation, resistive and conductive layers applied to the substrate using photoimaging and deposition techniques.
According to another aspect of the invention, pits are etched through the thin film structure into the wafer at one step. Ink feed holes are etched through the thin film structure and into the wafer, concurrently or during a separate step. At another step, material for an orifice layer is deposited into the pits and holes and onto the thin film structure. At another step, a firing chamber is etched into the orifice layer. During this step material is removed from the ink feed holes. At another step, a trench is etched into the backside of the wafer in the vicinity of the filled pits and the ink feed holes. The material filling each pit is not removed and remains in place to define the respective pillars. Two or more pillars are left protruding within the backside trench in the vicinity of the inlet channels for a corresponding nozzle chamber.
According to another aspect of the invention, an alternative fabrication process is used to from the pillars. After the thin film structure is applied, ink feed holes are etched into the thin film structure down into the substrate. Material for an orifice layer then is deposited into the holes and onto the thin film structure. A firing chamber then is etched into the orifice layer. During the etching of the firing chamber material is removed from the ink feed holes. At another step, a trench is etched into the backside of the wafer in the vicinity of the ink feed holes. After the trench is formed, a conforming layer of photoimagable material is spun into the trench along the backside of the substrate and thin film structure. At another step, an alignment and exposure process are performed to define an array of pillars within the trench. After the exposure, a developing process is performed to remove unwanted material and leave the pillars in place. The pillars are formed within the trench. Such pillars are formed on the underside of the thin film structure or on the backside of the substrate. In an alternative procedure, the pillars are formed before the orifice layer is deposited and the nozzle chamber is formed. One advantage of the photoimaging methodology embodiment is that the pillars can be formed to precise size and shape at desired locations.
According to another aspect of the invention, the pillars are formed prior to the step of applying the thin film structure to the printhead substrate. Pits are etched into the wafer at one step. At another step the pits are filled with a backside etchant-resistant material. The substrate then is planarized and fabrication continues with the deposition of the thin film layer and the orifice layer. The firing chamber, inlet channels and backside trench then are etched. During etching of the backside trench the etchant-resistant material filling the pits remains. Such material protrudes within the trench as the pillars. Two or more pillars are left protruding within the backside trench in the vicinity of inlet channels for a corresponding nozzle chamber.
One advantage of the invention is that pillars form a barrier xe2x80x98reefxe2x80x99 which keeps particles away from ink feed holes of nozzle chambers. Thus, fluid is able to flow into the nozzle chambers even in the presence of particles. Another advantage of the pillars is that ink drop weight is substantially unaffected and overshoot during refill is slightly reduced. A slight decrease in refill frequency is evident, however. These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.